# Earthquakes Chapter 11 P. Lobosco

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Earthquakes Chapter 11 P. Lobosco

Forces Inside the Earth Chapter 11, Section 1
Objectives: Explain how earthquakes result from the buildup of energy in rocks. Describe how compression, tension and shear forces make rocks move along the faults. Distinguish among normal, reverse and strike-slip faults.

Elastic Limit There is an elastic limit to the amount a rock can bend and stretch before it will break. Once the limit is passed, the rock will move along a fault line.

Earthquakes An earthquake is the shaking and trembling that results from the sudden movement of the rock along the fault. Energy is released. The earthquake will continue until the energy is used up.

Fault Line

Three Types of Force Compression is the force that squeezes rock together. Tension is the force that pulls rocks apart. Shearing is the force that causes rocks on either side of a fault to slide past each other.

Faults Tension causes rocks to be pulled apart and the rocks move down along the fault line. Compression causes rocks to squeeze together and rocks move up along a fault line. Strike slip is side by side.

Himalayan Mountains The Himalayan Mountains which were formed by the convergence (compression) of Indo-Australian Plate and the Eurasian Plate has many reverse and thrust faults.

Features of Earthquakes Chapter 11, Section 2
Objectives: Explain how earthquake energy travels in seismic waves. Distinguish among primary, secondary and surface waves. Describe the structure of Earth’s interior.

Seismic Waves The waves of energy generated by an earthquake are called seismic waves. There are three main types of seismic waves. Primary (P waves) Secondary (S waves) Surface (L waves)

San Andreas Fault The San Andreas fault extends 960 km from Mexico to the north of California. The land to the west is moving north. The land to the east of the fault is moving south. All the rocks do not move at the same time so earthquakes occur in one area and then another.

Focus Most faults occur between the surface and a depth of 70 kilometers. The point beneath the surface where the rocks break and move is called the focus. The focus is the underground origin of an earthquake.

Epicenter Directly above the focus, on the Earth’s surface is the epicenter. Earthquake waves reach the epicenter first. During an earthquake, the most violent shaking is found at the epicenter.

Primary Waves Seismic waves that travel fastest are P waves. They travel through solids, liquids and gases. They move at different speeds depending on the density of the material through which they are moving. As they move deeper in the Earth they move faster. P waves are push-pull waves.

Secondary Waves Seismic waves that do not travel through the Earth as fast as P waves do are called secondary or S waves. S waves travel through solids but not liquids or gases. S waves cause particles to move from side to side. They move at right angles to the direction of the wave.

P and S Waves

Surface Waves The slowest moving seismic waves are called surface waves or L waves. L waves originate on the Earth’s surface at the epicenter. They move along the surface the way waves travel in the ocean. The Earth’s surface moves up and down and side to side with each L wave. L waves cause most of the damage.

The Seismograph Invented in 1893 by John Milne, a seismograph detects and measures seismic waves. A weight attached to a spring remains nearly still even when the Earth moves. A pen attached to the weight records any movement on a roll of paper on a constantly rotating drum. The drum moves with the Earth and affects the line.

Seismograph

Seismologists Seismologists study earthquakes. They can determine the strength of an earthquake by the height of the wavy line recorded on the paper. The seismograph record of waves is called a seismogram.

Scales Used to Measure Earthquakes
The Richter scale is used to calculate the strength (magnitude) of an earthquake. The Modified Mercalli is used to measure the intensity (amount of destruction) of an earthquake.

Seismograph Stations Each type of wave reaches a seismograph station at a different time based on its speed. Primary waves arrive first. Secondary waves travel slower and arrive later. The difference in arrival time is used to estimate the distance from the station to the epicenter.

Locating an Epicenter Scientists need readings from three or more stations to determine the location. A circle is drawn around each station. The radius of each circle is equal to the station’s distance from the epicenter. The point of intersection is the location of the epicenter.

Interior of the Earth Seismic waves change speed as they move through the earth’s layers. They speed up when they pass through the bottom of the crust and enter the mantle. This boundary is known as the Moho.

The Moho The boundary is named for the scientist, Andrija Mohorovicic who discovered this information. It is called the Mohorovicic discontinuity or Moho.

Seismic Speed in the Mantle
The mantle is divided into layers based on changes in seismic wave speed. Both P and S waves slow down again when they reach the asthenosphere. They speed up when they move through a more dense part of the mantle.

Predicting Earthquakes
Scientists need to know the amount of stress applied as well as the elastic limit of the rock to determine when an earthquake will occur.

Predicting Earthquakes
Scientists have identified warning signals the help predict earthquakes with greater accuracy. Often changes occur in the speed of P and S waves before an earthquake occurs. Sometimes slight changes in the tilt of the Earth’s surface can be detected. Some scientists believe animals behavior is affected.

Seismic Speed in the Core
The core is divided into two layers based on how they travel through it. Primary waves slow down and secondary waves do not travel through the liquid outer core. Primary waves speed up again when they reach the solid inner core.

People and Earthquakes Chapter 11, Section 3
Objectives: Explain where most earthquakes in the United States occur. Describe how scientists measure earthquakes. Discuss earthquake safety procedures.

Seismic Risk Map of the U.S.

The Richter Scale measures the strength or magnitude
An increase of one magnitude on the Richter scale means that 32 times more energy is released. An earthquake of magnitude 6 is 32 x 32 x 32 times greater than an earthquake with a magnitude of 3.

Magnitude The height of the lines traced on the paper of a seismograph is a measure of the energy that is released, or the magnitude of an earthquake.

The Modified Mercalli Scale measures the amount of damage
Earthquakes can also be described by the amount of damage they cause. The Modified Mercalli scale describes the intensity of an earthquake using the amount of structural and geologic damage in a location.

Earthquake Damage The amount of damage created by an earthquake depends on several factors. The earthquake’s strength The kind of rock and soil that underlies an area The population of the area The kind of buildings in the area The time at which the earthquake occurs

Earthquake Belts Worldwide

Tsunamis Earthquakes which occur on the ocean floor produce giant sea waves called tsunamis. Tsunamis can travel at speeds of 700 to 800 km per hour. As they approach the coast, they can reach heights of greater than 20 meters. The Tsunami Warning Center is located in Hilo, Hawaii.

Earthquake Belts

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